This invention relates to a hydraulic damper of adjustable damping force type.
Recent automotive vehicles such as passenger cars tend to adopt hydraulic dampers of adjustable damping force type in suspension systems thereof. The driving comfort of the vehicles can be improved by changing damping force characteristics of hydraulic dampers in response to the condition of the road on which the vehicles are running and to the running speed thereof. Namely, when a vehicle is running on a smoothly paved road it is desired the reduce the damping force and when the vehicle is running on a rough road it is desired to increase the damping force.
Typically, such hydraulic dampers of adjustable damping force type comprise a piston working in a cylinder and partitioning the interior thereof into two liquid chambers, a piston rod secured to the piston and extending through one of the two liquid chambers and to the outside of the cylinder, a first passage formed through the piston for communicating the two liquid chambers, a damping force generating valve mechanism for controlling the liquid flow through the first passage, a by-pass passage formed in the piston rod for communicating the two liquid chambers separately from the first passage, and an adjusting valve operable from the outside of the cylinder and controlling the liquid flow through the by-pass passage. The adjusting valve acts to change the effective passage area of the by-pass passage, whereby the amount of the liquid flowing through the damping force generating valve mechanism and the damping force of the damper is adjusted both in the extension and contraction strokes of the damper. The by-pass passage constitutes an orifice of adjustable cross-sectional area and, thus, changes the increasing portion in the respective damping force-piston speed diagram. When the damping force increases to a predetermined level, the damping force generating valve on the piston opens, and the damping force is determined by the characteristics of the valve mechanism which is not substantially affected by the characteristics of the by-pass passage. In other words, the adjustment is only performed on the inclination of the increasing portion of the diagram, and is not performed on the damping force when the damping force generating valve is open.
Thus, it has recently been required to adjust the damping force of the hydraulic damper over the entire working range of the damper.